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The zombie fungus was never assigned any official taxonomic rank, nor was it ever scrutinized in the lab extensively enough to be compared to other observed species. What is know, however, is that it is almost completely unlike any other pathogen known to man. It is very complex in its workings and has a remarkably distinct genetic structure, supporting the hypothesis that it has long-since diverged from, or evolved entirely separately of all other life on Earth.

It is debatable as to how some of its features could have been completely evolutionarily advantageous, such as the way that it gestates in living organisms and its symptoms appear mostly after the host's death. This poses the possibility that it could have been engineered in a controlled environment, though this is doubtful due to the advanced understanding of gene-splicing that creating such a unique organism would have warranted.

Basic formes usually lack the fungal development to emit airborne spores, but contain spore-rich saliva that is capable of infecting through biting. Because of the poor hygiene of decaying zombies, their mouths, nails or claws, and some fungal growths may contain thriving deposits of bacteria, some lethal, which may facilitate the death of anyone who is directly injured by a zombie.

Some varieties, particularly Dispersers and Factories, have developed so that they make excellent vectors. They are capable of emitting spores capable of travelling further, or in a thick, gaseous forme. They may nurture airborne bacteria to potentially kill their victims shortly after infection.

During the gestation period it spends inside of living host, fungal spores gain foothold in the host's nervous system, whilst remaining dormant enough not to display any particularly irregular symptoms as long as the body remains living. During this period, it does, however, parasitize a small amount of the host's nutrient intake to support itself, and does have countermeasures to go unnoticed the host's immune system.

Once death occurs, the fungal network becomes metabolically demanding, and initiates an excessive consumption of dead fat-tissue and other non-essentials to begin expanding. Simultaneously, filamentous hyphae growths begin to creep along the pathways of the cadaver's circulatory system, metabolizing blood in its wake and laying the framework for what will be a new vascular system and hardpoints for more, individualized fungal growths to stem from later on, depending on the zombie's survival.

Because the ever-decaying biomass isn't enough to support a fungal colony, it has the ability to reanimate the necessary cells to bring a makeshift, feral sort of life and functional mobility back to the dead host that will then give it the means to metabolize more (thus zombies). In the process, the fungus targets specific areas of the brain not to receive respiration (effectively removing detrimental higher brain processes such as personality). To extend the zombie's effective lifespan, the fungus has the capability to denature hydrolytic enzymes to slow autolysis and measures of defending itself against decomposers.

The process of stabilizing the dead host's neural network was streamlined to take between four and six hours to prevent any irrevocable cytopathic decay in the necessary portions of the brain. The rewiring of the rest of the zombie's internal systems, however, typically takes anywhere from twelve to twenty-four hours, by which time a healthy zombie will be mobile.

Without zombies to consume flesh to feed it, these fungal colonies would typically starve themselves before they could reach the level of maturity to emit many spores.

The physiology of zombies varies greatly between different forms, though all zombies share core biological traits: most importantly its symbiotic relationship with the fungal colony inside of it, which actively stimulates nearly all of its functions. A mature fungal colony is composed of a main growth centered on the spinal cord below the neck, with tentacle-like mycelial cords of varying diameters branching out in all directions. These webs are interconnected through the zombie's tissue, typically purposed with taking-in and distributing nutrients, or producing spores that can be expelled from the body. The more developed zombie forms may possess external fungal masses with more "exotic" purposes, such as bolstering its offensive or defensive capabilities.

The fungal matter is of a pale green to light brown coloration, and has a consistently mottled, chitinous texture. Internal fungal masses are mostly soft and moist, while growths that develop externally on the skin tend to be more dense with a hard, calcified encasing.

The nature of the fungal colonies leads to an interesting display of polymorphism in its zombies. Due to gradual decay and their compromised physical state, all zombies come with a shelf life during which they can remain active. The general rule is that the more flesh a zombie devours, the more energy it harnesses to polymorphize. Zombies that have the means to devour large-quantities of flesh and metabolize, progress into more diverse, specialized forms that tend to focus on killing and eating. Parallels can be drawn between the polymorphism of zombies and natural selection, as "survival of the fittest" applies to both concepts.

The far majority of zombies never have the chance to progress into different forms. Zombies that are only efficient enough to focus on infecting others, eventually degrade into immobile, overgrown masses that act as airborne spore dispersal units. The majority of zombies are of the lesser variety, with their typically being only a few advanced forms per horde. The advanced forms have an enormous potential to become lethal, and are the most capable of decimating survivor resistance and breaching strongholds, making way for hordes of lesser zombies to infect, and giving them more chances to morph themselves.

To increase their effectiveness, zombies have the ability form hordes or packs. As with other social animals, a zombie horde requires a designated leader, or alpha, which is typically the most fit or most well-developed. Alphas aren't of any specific form, rather they are responsible for coordinating group movements and performing the most communication and thus is required to be particularly intact.

To protect their alpha, a developing horde will usually encircle around their leader, ever-growing as they recruit lone zombies. When multiple hordes encounter each other, they may act in unison or converge around the superior zombie from both their groups.

A developed fungal infection will begin emitting pheromones, broadcasting to other fungal infections as a sort of friend or foe identification. This enables other zombies to recognize the integrated individual as one of them and know not to attack it. These hormones are later used to encourage horde development, synchronize and coordinate group movements. Horde leaders, the designated alphas, will usually secret the most pheromones and make vocal noises frequently, allowing lesser zombies to identify and know to converge around them.